To update, add the following repository to your software sources list:

ppa:kubuntu-ppa/backports

or if it is already added, the updates should become available via your preferred update method.

The PPA can be added manually in the Konsole terminal with the command:

sudo add-apt-repository ppa:kubuntu-ppa/backports

and packages then updated with

sudo apt updatesudo apt full-upgrade

Upgrade notes:

~ The Kubuntu backports PPA includes various other backported applications, so please be aware that enabling the backports PPA for the first time and doing a full upgrade will result in a substantial amount of upgraded packages in addition to Plasma 5.11.5.

~ The PPA may also continue to receive updates to Plasma when they become available, and further updated applications where practical.

~ While we believe that these packages represent a beneficial and stable update, please bear in mind that they have not been tested as comprehensively as those in the main Ubuntu archive, and are supported only on a limited and informal basis. Should any issues occur, please provide feedback on our mailing list [1], IRC [2], and/or file a bug against our PPA packages [3].